This invention is directed to a continuous process for the manufacture of elastomer-modified monovinylaromatic polymers such as high impact polystyrene (HIPS). Rubber-reinforced polymers of monovinylaromatic compounds, such as styrene, alphamethyl styrene and ring-substituted styrenes are desirable for a variety of uses. More particularly, rubber reinforced polymers of styrene have included therein discrete particles of a crosslinked rubber, for example, polybutadiene, the said discrete particles of rubber being dispersed throughout the styrene polymer matrix and in addition said particles of rubber contain polystyrene inclusions. HIPS can be used in a variety of applications including refrigerator linings, packaging applications, furniture, household appliances and toys. Such HIPS are described in WO 01-68765, EP 1148086, U.S. Pat. No. 6,825,270, EP 770632, EP 1251143, EP 620236, US 2005-0070662, U.S. Pat. No. 6,569,941 and EP 664303.
The process for making HIPS is well known to those skilled in the art and consists of polymerizing styrene monomer in the presence of dissolved rubber. Polymerization of styrene, and optionally a comonomer, is initiated by heating and/or by an initiator, by way of example a radical initiator. The rubber is “dissolved” in the styrene monomer (actually the rubber is infinitely swollen with the monomer). The usual rubber types utilized in the manufacture of HIPS include polybutadiene (PB), styrene-butadiene rubber (SBR), and styrene-butadiene-styrene rubber (SBS). Polystyrene is initially formed from the styrene monomer within the homogeneous rubber solution in styrene. At the beginning of the polymerization the reacting solution is at a point prior to the rubber/styrene inversion point, i.e. the point at which the solution being reacted goes from polystyrene particles in a rubber/styrene monomer matrix to rubber particles in a polystyrene matrix. When the degree of polymerization is about equal to the weight % of rubber in the system, it inverts e.g. the styrene/styrene polymer phase becomes continuous and the rubber phase becomes discontinuous. Styrene is polymerized around and within the rubber particles which leads to polystyrene inclusions in the rubber particles. A portion of the styrene is polymerized by grafting on the rubber, another portion is homopolymerized, said portion is referred to as a “non-grafting” polymerization. In HIPS a part of the styrene may be replaced by unsaturated monomers copolymerizable with styrene such as other monovinylaromatic monomers, alkyl esters of acrylic or methacrylic acid and acrylonitrile. The same mechanism of “grafting” and “non-grafting” occurs with the styrene comonomer, which means one portion of the styrene and of the comonomer are polymerized by grafting on the rubber, another portion of the styrene and of the comonomer are copolymerized. The properties of HIPS are related to the amount of rubber, the type of rubber, the rubber particles size as well as the polystyrene included in the rubber particles. The proportion of styrene, and the optional comonomer, which is grafted (polymerized by the “grafting” way) is linked to the rubber particles size as well as to the amount of polystyrene included in the rubber particles.
A lot of prior art has already described such processes.
EP 1251143 A1 describes a method for improving the environmental stress crack resistance of an elastomer-modified monovinylaromatic polymer material, comprising:                introducing a monovinylaromatic monomer feed stream into a polymerization reactor;        introducing an elastomer feed stream into said polymerization reactor;        introducing a polymerization initiator compound into said reactor, said initiator compound comprising at least one perketal in amounts of about 200 parts per million (ppm), by weight, and at least one peroxycarbonate in amounts ranging from about 150 to about 800 ppm, by weight;        said perketal comprises Lupersol L-231 and said peroxycarbonate comprises t-Amyl 2-Ethylhexyl peroxycarbonate; and        reacting said monomer, said initiator compound, and elastomer to form an elastomer-modified monovinylaromatic polymer having high ESCR value.This combination of initiators increases the grafting level.        
EP1245599A2 relates to a process for the continuous production of high impact polystyrene wherein styrene monomer is polymerized in the presence of an elastomer dissolved therein, said process having at least one continuous-stirred tank reactor, the improvement comprising utilizing as an initial reactor, an elongated stirred tank reactor having plugflow characteristics, and wherein said elongated stirred tank reactor has a reaction zone having a length to diameter ratio exceeding about 2 and utilizes advantageously an upflow configuration. Advantageously there is an absence of a continuously operating preheater located prior to the elongated reactor. Advantageously an initiator in amounts of at least 100 ppm are utilized in the styrene monomer/elastomer solution in said elongated reactor. Advantageously said elongated reactor reaction zone has a height to diameter ratio of about 3 to about 4. Advantageously said elongated reactor comprises at least three reaction zones, a bottom preinversion zone, a middle inversion zone, and an upper heat vaporization zone. Grafting is calculated from the gel to rubber ratio. The percent gel is measured by first dissolving the resin in toluene, separating the gel fraction by centrifugation, and then drying the wet gel. The percent gel is then calculated from this dried residue by the formula: Percent Gel=100×dried gel weight, divided by the initial weight of the sample. The percent rubber is measured by the Iodine Monochloride (I—Cl) titration method.
WO 2005 033176 relates to a process for preparing a high impact polystyrene comprising admixing a rubber and styrene monomer in the presence of at least two polymerization initiators and polymerizing the styrene wherein at least one of the at least two polymerization initiators is a grafting initiator and at least one of the at least two polymerization initiators is a non-grafting initiator. The rubber may be selected from the group consisting of polybutadiene, styrene-butadiene rubber, styrene-butadiene-styrene rubber, natural rubber, and mixtures thereof. The grafting initiator is advantageously selected from the group consisting of 1,1-di-(t-butylperoxy)cyclohexane; 1,1-di-(t-amylperoxy)cyclohexane); 1,1-di-(t-butylperoxy)-3,3,5-trimethyl-cyclohexane; 00-t-amyl-0-(2-ethylbexyl monoperoxy-carbonate); OO-t-butyl O-isopropyl monoperoxy-carbonate; OO-t-butyl-0-(2-ethylhexyl)monoperoxy-carbonate; butyl 4,4-di(t-butylperoxy)valerate; Ethyl 3,3-Di-(t-butylperoxy)butyrate; and mixtures thereof. The non-grafting initiator is advantageously selected from the group consisting of 2,2′-azobis(isobutyronitrile), 2,2′-azobis(2-methylbutyronitrile), lauroyl peroxide, decanoyl peroxide, and mixtures thereof.
JP 20053305446 A published on 2 Dec. 2005 describes styrene polymerization in the presence of oxygen and amine boranes, but polymerization is made without rubber.
It has been discovered that the use of boranes complexes as initiators in the process for the production of elastomer-modified high-impact monovinylaromatic polymers increases the ratio of grafted monovinylaromatic monomer to the non-grafted monomer.